1. Field of the Invention
The present invention relates to a self-pinned spin valve sensor with a high coercivity antiparallel (AP) pinned layer and, more particularly, to a first AP pinned layer which is farther from a spacer layer than a second AP pinned layer and is composed of a material which has high coercivity to prevent flipping of the magnetizations of the AP pinned layers and high resistivity to prevent sense current shunting.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a magnetic disk, a slider that has read and write heads, a suspension arm and an actuator arm that swings the suspension arm to place the read and write heads adjacent selected circular tracks on the disk when the disk is rotating. The suspension arm biases the slider into contact with the surface of the disk or parks it on a ramp when the disk is not rotating but, when the disk rotates and the slider is positioned over the rotating disk, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic field signals to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from the rotating magnetic disk, which sensor is also known as a giant magnetoresistance (GMR) sensor. The spin valve sensor comprises a nonmagnetic electrically conductive spacer layer that is sandwiched between a ferromagnetic pinned layer and a ferromagnetic free or sense layer. An antiferromagnetic pinning layer typically interfaces the pinned layer for pinning the magnetization of the pinned layer 90° with respect to an air bearing surface (ABS) of the sensor wherein the ABS of the sensor is an exposed surface of the sensor that faces the rotating disk. First and second hard bias and lead layers are typically connected to the sensor for conducting a sense current therethrough. The magnetization of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative signal fields respectively from the rotating magnetic disk. The quiescent position of the magnetization of the free layer, which is parallel to the ABS, is when the sense current is conducted through the sensor without signal fields from the rotating magnetic disk.
The spin valve sensor is located between nonmagnetic first and second electrically nonconductive first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. The distance between the first and second shield layers defines the linear bit density of the read head and the track width (TW) of the spin valve sensor at the free layer defines the track width density of the read head. These combined densities, which is known as areal density, governs the storage capacity of the magnetic disk drive. First and second hard bias and lead layers are typically connected to first and second side surfaces of the sensor, which connection is known in the art as a contiguous junction. This junction, which conducts the sense current (IS) through the sensor, is described in commonly assigned U.S. Pat. No. 5,018,037. The first and second hard bias layers longitudinally stabilize the magnetization of the free layer of the sensor in a single domain state which is important for proper operation of the sensor.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor to the sense current (IS) is at a minimum and when their magnetic moments are antiparallel the resistance of the sensor to the sense current (IS) is at a maximum. Changes in resistance of the sensor is a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layers. When the sense current (IS) is conducted through the sensor, resistance changes, due to signal fields from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified by a giant magnetoresistance (GMR) coefficient ΔR/R where ΔR is the change in resistance of the read sensor from minimum resistance (when magnetizations of free and pinned layers are parallel to each other) to maximum resistance (when magnetizations of the free and pinned layers are antiparallel to each other) and R is the resistance of the read sensor at minimum resistance.
Spin valve sensors are classified as a bottom spin valve sensor or a top spin valve sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel (AP) pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or first and second (AP1) and (AP2) ferromagnetic AP layers that are separated by a coupling layer with magnetic moments of the ferromagnetic AP layers being antiparallel. The advantage of the AP pinned layer structure is that the magnetizations of the first and second AP pinned layers (AP1) and (AP2) substantially counterbalance one another so that the net magnetization of the AP pinned layer structure minimally affects the quiescent parallel position of the free layer. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
A scheme for minimizing the aforementioned gap length between the first and second shield layers is to provide a self-pinned AP pinned layer structure. The self-pinned AP pinned layer structure eliminates the need for the aforementioned pinning layer which permits the read gap to be reduced. In the self-pinned AP pinned layer structure each of the first and second AP pinned layers has a uniaxial anisotropy field due to crystalline structure and other factors concerning the material and a magnetostriction uniaxial anisotropy field where the magnetostriction uniaxial anisotropy field is equal to
      3    ⁢    λσ        magnetization    ⁢                  ⁢    of    ⁢                  ⁢    the    ⁢                  ⁢    layer  where λ is magnetostriction and σ is stress of the layer. A positive magnetostriction of the layer and compressive stress therein results in a magnetostriction uniaxial anisotropy field that can support the uniaxial anisotropy field. Compressive stress in the AP pinned layers occurs when the magnetic head is lapped to the ABS. The orientations of the magnetic moments of the AP pinned layers are set by an external field.
In the AP pinned layer structure the second AP pinned layer (AP2) is located between the first AP pinned layer (AP1) and the spacer layer. The first AP pinned layer (AP1), which is farther away from the spacer layer than the second AP pinned layer (AP2), can be referred to as a keeper layer since it keeps the magnetization of the second AP pinned layer (AP2) aligned perpendicular to the ABS and the second AP pinned layer (AP2) can be referred to as a reference layer since it is the relative orientation of the magnetizations of the free layer and the second AP pinned layer (AP2) that determine the resistance of the spin valve sensor. While the AP pinned layer structure reduces the net magnetic moment on the free layer structure and the self-pinned AP pinned layer structure reduces the read gap, the keeper layer shunts a portion of the sense current which is not the case when the single pinned layer is employed for the pinned layer structure. Accordingly, there is a strong-felt need to reduce the sense current shunting through the keeper layer. Another problem is that if the self-pinning of the AP pinned layer structure is not sufficient, unwanted extraneous fields can disturb the orientations of the magnetic moments of the AP pinned layers or, in a worst situation, can reverse their directions. This is known in the art as amplitude flipping.